For example, in a wound-field rotating electrical machine for vehicle in the related art, a field current is supplied from a brush to a field winding of a rotor via a slip ring. In this instance, the brush slides over the slip ring while the brush is pressed against the slip ring by a spring attached behind the brush. The brush and the slip ring generate heat due to an electrical loss and sliding friction in a sliding portion and an electrical loss in an energizing path of each. A set of the slip ring and the brush is installed on each of a plus side and a minus side. These brushes and slip rings are generally covered and held in a brush holder made of insulating resin or the like to ensure electrical isolation between the plus side and the minus side.
A labyrinthine structure is provided between the brush holder storing the brushes therein and mold resin of the slip rings. The labyrinthine structure prevents entry of foreign matter and moisture from the outside and is also furnished with a brush abrasion powder discharging function. A slight clearance is provided to the labyrinthine structure. However, only a small proportion of cooling air generated by a cooling fan provided to the rotor is supplied to an abutment portion of the slip ring and the brush. Because of these factors, the slip ring and the brush become hot chiefly in the abutment portion. When the brush becomes hot, a wear rate becomes higher, which raises a problem that the life of the brush becomes shorter.
In a rotating electrical machine like a motor generator that performs not only power generation during the driving but also regenerative power generation and a start-up operation, a current exceeding 10 amperes (A) is passed through the brushes for a period of less than a second to about tens of seconds in some cases. In such a case, a large amount of heat is generated at the brushes and the temperature rises abruptly. As a result, the brush holder holding the brushes becomes hotter than its melting temperature and may possibly cause a failure.
In order to eliminate this problem, there is a structure by which heat generated at the brushes is released to the outside by heat transfer using radiation fins attached to an outer portion of the brush holder (for example, PTL 1). Alternatively, there is proposed a structure in which the brushes are held by a brush holder made of metal and plates that supply a field current to the brushes integrally molded with a brush holder base holding the brush holder are connected to a bracket via a sealing material, so that heat generated at the brushes is released to the bracket (for example, PTL 2). Further, there is proposed a structure in which ventilation openings are provided to the brush holder to cool the brushes and to discharge brush abrasion powder, so that the temperature of the brushes is lowered and hence the life of the brushes is extended (for example, PTL 3).